Radiation therapy consists of the use of ionizing radiation to treat living tissue, usually tumors. There are many different types of ionizing radiation used in radiation therapy, including high energy x-rays, electron beams, and proton beams. However, the process of administering the radiation to a patient can be somewhat generalized regardless of the type of radiation used. According to conventional radiation therapy, a beam of radiation is directed toward a tumor located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the tumor according to a treatment plan. The delivered radiation kills cells of the tumor by causing ionizations within the cells. However, use of a single pre-treatment scan can lead to a large planning target margin and uncertainty in normal tissue dose due to patient variations, such as organ movement, shrinkage and deformation, which can occur from the start of a treatment session to the end of the treatment session.
The radiation therapy techniques include the use of Intensity Modulated Radiotherapy (“IMRT”), typically by means of a radiotherapy system, such as a linear accelerator, equipped with a multileaf collimator (“MLC”). Use of multileaf collimators in general, and IMRT in particular, allows the radiologist to treat a patient from multiple angles while varying the shape and dose of the radiation beam, thereby providing greatly enhanced ability to deliver radiation to a target within a treatment volume while avoiding excess irradiation of nearby healthy tissue. The recently developed and clinically adopted technique known as volumetric modulated arc therapy (VMAT) improves target conformity and organ sparing by use of rotational intensity modulated radiation therapy (IMRT) and more control points (gantry locations) for intensity optimization (McGrath S D, Matuszak M M, Yan D, Kestin L L, Martinez A A, Grills I. Volumetric modulated arc therapy for delivery of hypofractionated stereotactic lung radiotherapy: A dosimetric and treatment efficiency analysis. Radiother Oncol 2010; 95:153-7; Matuszak M M, Yan D, Grills I, Martinez A. Clinical applications of volumetric modulated arc therapy. Int J Radiat Oncol Biol Phys 2010; 77:608-16). VMAT is a new type of intensity-modulated radiation therapy (IMRT) treatment technique that uses the same hardware (i.e. a digital linear accelerator) as used for IMRT or conformal treatment, but delivers the radiotherapy treatment using rotational or arc geometry rather than several static beams. This technique uses continuous modulation (i.e. moving the collimator leaves) of the multileaf collimator (MLC) fields, continuous change of the fluence rate (the intensity of the X rays) and gantry rotation speed across a single or multiple 360 degree rotation(s). This significantly reduces beam delivery time compared to conventional fixed field IMRT (otherwise known as step and shoot IMRT). During a VMAT treatment, the Linear Accelerator rotates around the patient while the radiation beam is shaped and reshaped as it is continuously delivered from virtually every angle in a revolution. During a VMAT treatment, specialized software algorithms will vary the three parameters simultaneously: the speed of rotation around the patient, the shape of the MLC aperture, and the dose delivery rate. The target volume dose does not change when using VMAT. The amount of scatter and leakage radiation dose to the rest of the body is reduced compared to conventional IMRT. Varian (Varian Medical Systems, Palo Alto, Calif., USA) develops a VMAT product marketed as RapidArc. The US FDA approved RapidArc for clinical use in February 2008. In its first released software version, only one or two full rotation arcs could be planned. In the second software upgrade (Aria version 8.6) released in the first half of 2009, partial arcs, arcs with exclusion zones (e.g. so that the entry angle through a metallic hip replacement can be avoided) and arcs from different gantry angles (e.g. vertex fields for cranial treatments) allowed greater freedoms of dose intensity modulation for complex target volumes where adjacent critical normal tissue structures need to be avoided.
Elekta (Elekta AB, Stockholm, Sweden) also have a product named VMAT, which does not use Otto's algorithm, but uses a proprietary algorithm. This emphasized multiple arcs from the earliest software releases, in contrast to the early Varian releases. The planning technique for VMAT has evolved with software upgrades. When first introduced, a plan using a double arc to treat a 2 Gray planning target volume (PTV), the first arc optimization is dosed to 1 Gray. The second arc is then optimized to the existing single arc plan, with the smoothing and filling of cold spots and the cooling of hot spots, leading to a more homogenous PTV dosing. With the latest software versions, the planner defines two arcs with starting and stopping positions, and then the optimization occurs to the full 2Gy to the PTV. The VMAT optimization is a two-step process. A set of ideal intensity maps is generated first—this takes 10-20 minutes. A leaf sequencing process where leaves move smoothly between adjacent arc segments follows this. This process used to take 20 minutes but has now been substantially shortened by employing four quad processors to optimize four arc segments simultaneously. Additional boost volumes can be added with a second or third arc—allowing concomitant boosts or field in field effects. The additional arc may also provide supplementary aperture shape variation for a complex dose distribution.
U.S. Pat. No. 8,027,431 provides a system and method to receive a radiation treatment plan for delivering at least a portion of a prescribed radiation dose to a target volume in a series of individual treatment beams in an arc around the target volume, each individual treatment beam having a start angle and a stop angle; and deliver a portion of the prescribed radiation dose to the target volume over each of the segments, the segments arranged in a contiguous manner on the arc and the delivery of the prescribed radiation dose is continuous through the segments. However, there has not been a solution to breath-hold coordination for the dynamic delivery of the VMAT system (for example, Eleka VMAT, Elekta Oncology System Ltd., Crawley, West Sussex, UK).
The delivery of each arc of VMAT usually takes more than 1 to 2 minutes, longer than a single tolerable breath hold. Breath hold and respiratory gating are two established strategies for reducing respiration-induced organ motion in radiotherapy. Deep-inspiration breath hold is a controlled breathing technique in which the patient performs a supervised breath hold during radiotherapy, with the dual benefits of reduced respiratory motion from the breath hold and increased normal tissue sparing from the increased tissue volume. Respiratory gating depends on a device external to the patient monitoring breathing and allows delivery of radiation only during certain time intervals, synchronous with the patient's respiratory cycle. Gated radiotherapy requires less patient effort than breath hold, but has more organ motion than static breath hold. Respiration-induced dose-delivery errors are demonstrated with a realistic breathing lung phantom, with exceptionally significant errors by single-arc VMAT using a high dose rate (Court L E, Seco J, Lu X Q, Ebe K, Mayo C, Ionascu D et al. Use of a realistic breathing lung phantom to evaluate dose delivery errors. Med Phys 2010; 37:5850-7). Efforts have been made to develop gating solutions to VMAT delivery. Varian's TrueBeam™ (Varian Medical Systems, Palo Alto, Calif., USA) first supported gated VMAT by responding a gating signal from a real-time position management (RPM™) system. Qian et al. adapted a log-file-based dose reconstruction and verified the fidelity of gated VMAT delivery for three patients with lung or pancreatic tumors with three simulated respiratory periods (Qian J, Xing L, Liu W Luxton G. Dose verification for respiratory-gated volumetric modulated arc therapy. Phys Med Biol 2011; 56:4827-38). Preclinical evaluation of Varian's gated RapidArc delivery by use of 2-dimensional dose verification was satisfactorily conducted (Nicolini G, Vanetti E, Clivio A, Fogliata A, Cozzi L. Pre-clinical evaluation of respiratory-gated delivery of volumetric modulated arc therapy with RapidArc. Phys Med Biol 2010; 55:N347-57). All the work focuses on Varian's VMAT and RPM™ gating systems, which use the signal of chest wall movement to represent respiratory oscillation and involve complex interactions between MLC kinetics, dose-rate modulation, and gantry rotation. A 4-dimensional VMAT planning framework is under investigation, with the contributions of beams and organ motion from different breathing phases integrated into the optimization process. However, such a strategy is more theoretical than practical for fractionated treatment (Chin E, Otto K. Investigation of a novel algorithm for true 4D-VMAT planning with comparison to tracked, gated and static delivery. Med Phys 2011; 38:2698-707). Radiotherapy delivered by Elekta's linear accelerator has used the breath-hold strategy, either by active breathing coordination or passive abdominal compression, to reduce respiration-induced dose errors. Therefore, breath-hold timing and interval are more voluntary and predictable for gated VMAT. To deliver dynamic VMAT within the breath-hold intervals, segmented short arcs of less than 20 to 30 seconds each are required. The available treatment planning systems either have a minimum requirement of gantry rotation range for the arc design, such as ≧90° arc with the Pinnacle system, or have less satisfactory planning results with short arcs (Bertelsen A, Hansen C R, Johansen J, Brink C. Single Arc Volumetric Modulated Arc Therapy of head and neck cancer. Radiother Oncol 2010; 95:142-8). To our knowledge, there has not been any gating solution to VMAT delivery by Elekta's accelerator (Dobler B, Groeger C, Treutwein M et al. Commissioning of volumetric modulated arc therapy (VMAT) in a dual-vendor environment. Radiother Oncol 2011; 99:86-9; Kida S, Saotome N, Masutani Y et al. 4D-CBCT reconstruction using MV portal imaging during volumetric modulated arc therapy. Radiother Oncol 2011; 100:380-5).
Therefore, there is still a need to find a solution to solve the problem in taking the time longer than a single tolerable breath hold to deliver each arc of VMAT.